1. Field of Invention
This disclosure relates in general to connections of mineral insulated metal sheathed (MIMS) cables and a method for forming the connection. More specifically, the disclosure relates to a compression fitting used for splicing together ends of MIMS cable.
2. Description of Prior Art
Mineral insulated cables are used for conducting electricity either to provide power to a separate component or for heating the cable itself as a heating element. Mineral insulated cables are also used for sensing ambient conditions, such as temperature or pressure. The mineral insulation enables MIMS use in harsh environments, such as extreme temperature. Typically, the outer surface or outer sheath of the mineral insulated cables (MIMS) is comprised of a high temperature metal, such as stainless steel. MIMS cable assemblies typically comprise a conductive member or conductive element (such as a wire) covered with mineral insulation. The mineral insulation typically is magnesium oxide (MgO). Magnesium oxide has been chosen as the insulation material since it exhibits stability at high temperatures and it does not react with either the conductive element or the metal sheath.
MIMS cables are formed by inserting the conductive element within a metal tube then adding magnesium oxide to the annulus between the wire and tube. The combination is then either swaged or pulled through a reduced diameter element, such as a die, thereby reducing its diameter and compressing the tube and insulation tightly around the wire to form a cohesive unit.
MIMS cables are used for many applications where conductors inside the cable must be protected from the harsh and ambient environment and insulated from one another and from the sheath. These applications include electrical and instrumentation cables, thermalcouple, and RTD cables exposed to chemical processes and other harsh conditions. Additionally, resistance type cables may also be employed with this cable that operate up to high temperatures. It is required from time to time to splice MIMS cables together, either to repair damaged cable or to add components in line, as well as the need to construct a long length of cable from shorter pieces. Care must be taken when forming these splices since the magnesium oxide is quite hygroscopic and absorbs moisture when exposed to ambient conditions. Moisture trapped in the cable can reduce both its thermal and electrical insulating effectiveness directly and can degrade the magnesium oxide also adversely affects its insulating properties. Accordingly, the performance of the cable would be affected by moisture content within the magnesium oxide or other insulating materials that might be used.
Splicing kits are available for MIMS cables. However, the kits are specific to certain types of cables and usually not effective in maintaining the original properties of the cable after the sheath has been breached. The cable will lose its effectiveness or deteriorate more quickly if the electrical, thermal, or mechanical properties of the cable are compromised. For example, a contaminated MIMS cable has a reduced voltage capacity and is prone to inducing a short in the circuit. Similarly, damaged MIMS cables associated with sensing devices will affect the voltage output thereby compromising the efficacy of the sensing unit. Accordingly, room for improvement exists in methods for providing splices in mineral insulated metal sheathed cable assemblies.